Multilayer, microporous polyolefin membranes have been used in various applications such as separators for batteries, separators for electrolytic capacitors, various filters, moisture-permeable waterproof clothing, reverse osmosis filter membranes, ultrafiltration membranes, and microfiltration membranes. When a multilayer, microporous polyolefin membrane is used as a separator for a battery, in particular as a separator for a lithium ion battery, the performance thereof is closely related to characteristics, productivity, and safety of the battery. Therefore, excellent permeability, mechanical properties, heat shrinkage resistance, shutdown characteristics, melt down characteristics, and the like are required. For example, if a separator for a battery, the separator having low mechanical strength, is used, the voltage of the battery may be lowered due to a short circuit of electrodes. Furthermore, since it is known that the battery performance of a lithium ion battery is deteriorated when the lithium ion battery is repeatedly charged in the state that it is almost fully charged. Since oxidative deterioration of the separator can be a part of the cause, improvement in separators has been demanded.
As the methods for improving the physical properties of microporous polyolefin membrane, improvements in raw material compositions, stretching conditions, heat treatment conditions, and the like have been investigated and, as a method of enhancing heat resistance, blending of polypropylene has been proposed (e.g. Japanese Unexamined Patent Application Publication No. 2002-105235 and Japanese Unexamined Patent Application Publication No. 2003-183432). In particular, characteristics related to battery productivity, such as electrolyte injection performance, and characteristics related to battery life, such as oxidation resistance, are regarded as important recently in addition to permeability, mechanical properties, heat shrinkage resistance, and the like.